Examples of hydraulically-actuated electronically-controlled unit injectors are shown in U.S. Pat. No. 3,689,205 issued to Links on Sep. 5, 1972, U.S. Pat. No. 4,271,807 issued to Links et al. on June 9, 1981, and U.S. Pat. No. 4,605,166 issued to Kelly on Aug. 12, 1986.
The viscosity of the actuating fluid used to actuate an intensifier piston in this type of unit injector normally varies with ambient temperature and affects the magnitude of pressure drops in the actuating fluid circuit. In order to achieve quick startup of an engine equipped with such unit injectors, the length of time or pulsewidth of a fuel delivery command signal to the electronic actuator of the unit injector and/or the actuating fluid pressure should be higher when the viscosity of the actuating fluid is relatively high. The viscosity of the actuating fluid is highest under cold engine starting conditions. On the other hand, after the engine has started and the actuating fluid warms up, the pulsewidth of the fuel delivery command signal and/or the actuating fluid pressure should be relatively lower in order to improve governability of the engine and/or avoid producing excessive engine torque which may damage the engine. In the absence of directly or indirectly detecting the viscosity of the actuating fluid, it is difficult to select the proper pulsewidth of the fuel delivery command signal and/or actuating fluid pressure which avoids undesirable results to the engine. If the selected pulsewidth and/or pressure is too low, insufficient or no fuel may actually be injected by the unit injector which will inhibit starting or continued operation of the engine. If the selected pulsewidth and/or pressure is too high, an excessive amount of fuel may be injected by the unit injector which may cause damage to the engine.
The present invention is directed to overcoming one or more of the problems as set forth above.